Nano-structure arrays for EMR imaging

ABSTRACT

An detector material for detecting electro-magnetic radiation is described. The detector material includes a substantially regular array of nano-particles embedded in a matrix material. The nano-particles are arranged such that when a bias voltage to the matrix material is applied, electrical current is directly generated based on a cooperative plasmon effect in the detector material when electro-magnetic radiation in a predetermined wavelength range is incident upon the detector material, where the dominant mechanism for decay in the cooperative plasmon effect is non-radiative.

FIELD OF THE INVENTION

The present invention relates to the use of nanoparticle arrays embeddedin a matrix material for the detection of electromagnetic radiation(EMR), EMR detector materials employing such nanoparticle arrays, andEMR detectors employing such detector materials.

BACKGROUND

Nanoparticles are widely used for their optical properties. In theiruse, nanoparticles are known to be excellent scatterers of light andother EMR.

Gold nanoparticles have been used to absorb EMR to create a rise intheir temperature which can subsequently be measured through resistancechange in their surrounding medium. Such gold nanoparticles can be thebasis of potential microbolometer improvement [Nikoobakht, B., May 272010; US Patent Application Publication US 2010/0127172 A1]. In suchsystems, however, no direct use is made of the electrical properties ofthe plasmon's electron cloud at the surface of the nanoparticles, andthe electric field the plasmon's electron cloud creates. Electrons arenot detected directly as current in such systems.

Nanoparticles have also been attached to the surface of silicon solarcells for the purposes of scattering (re-radiating) the incident EMRfrom the Sun into the optical modes of a photovoltaic silicon detector,for the purposes of increasing absorption of EMR in the silicon tocreate an electric current flow, such as is the standard process inoptoelectronic photodetection devices [Fonash, S. J., 2010; ‘Solar CellDevice Physics’, Elsevier.].

Indeed, there is considerable global interest at present in the emergingfields of plasmonics, photonic crystals and so called meta-materials[Wehrspohn, R. B., Kitzerow, H.-S., and Busch, K., 2008; NanophotonicMaterials: Photonic Crystals, Plasmonics, and Metamaterials',Wiley-VCR]. Throughout the development of these materials, the focus hasbeen on the manipulation of their optical and non-linear opticalproperties for a variety of optical applications.

SUMMARY

According to one embodiment, there is provided a detector material fordetecting electro-magnetic radiation, comprising: a substantiallyregular array of nano-particles embedded in a matrix material; and thenano-particles arranged such that when a bias voltage to the matrixmaterial is applied, electrical current is directly generated based on acooperative plasmon effect in the detector material whenelectro-magnetic radiation in a predetermined wavelength range isincident upon the detector material, where the dominant mechanism fordecay in the cooperative plasmon effect is non-radiative.

According to one aspect of the embodiment, the nano-particles comprise ametal, and the matrix material comprises a semiconductor material.

According to another aspect of the embodiment, the nano-particlescomprise gold.

According to another aspect of the embodiment, the matrix materialcomprises one of silicon or InSb.

According to another aspect of the embodiment, the nano-particlescomprise a semiconductor material, and the matrix material comprises ametal.

According to another aspect of the embodiment, the nano-particlescomprise a first semiconductor material, and the matrix materialcomprises a second semiconductor material.

According to another aspect of the embodiment, the second semiconductormaterial is highly doped.

According to another aspect of the embodiment, the nano-particlescomprise a semiconductor material, and the matrix material comprises aconductive contact material having ohmic or Schottky barrier propertieswith respect to the embedded nano-particles.

According to another aspect of the embodiment, the matrix materialcomprises a heterojunction, and the nano-particles are embedded in theheterojunction.

According to another aspect of the embodiment, the matrix materialcomprises a metal-semiconductor junction, and the nano-particles areembedded in the metal-semiconductor junction.

According to another aspect of the embodiment, the matrix materialcomprises a metal as a perforated contact electrode and thenano-particles comprise a semiconductor material.

According to another aspect of the embodiment, nano-particles arecylindrical, spherical, cubic, rectangular-cubic, ellipsoidal, planar orspiral-twisted.

According to another aspect of the embodiment, the nano-particles have along axis, and the array comprises one or more pixels.

According to another aspect of the embodiment, the nano-particles ineach pixel are arranged such that the long-axis of each nano-particle inthe pixel are oriented along a same direction.

According to another aspect of the embodiment, the nano-particles ineach pixel comprise a first group of nano-particles and a second groupof nano-particles, the long-axis of each nano-particle in the firstgroup oriented along a first direction, the long-axis of eachnano-particle in the second group oriented along a second direction.

According to another aspect of the embodiment, the first direction isperpendicular to the second direction.

According to another aspect of the embodiment, the nano-particles of thefirst and second groups are arranged in the same region of the pixel toform a checker-board pattern.

According to another aspect of the embodiment, the nano-particles of thefirst and second groups are arranged in different regions of the pixel.

According to another aspect of the embodiment, the nano-particles ineach pixel comprise a first group of nano-particles arranged in a firstregion of the pixel and a second group of nano-particles arranged in asecond region of the pixel, wherein the nano-particles in the firstgroup are arranged in the first region to optimally detectelectromagnetic radiation in a first wavelength range, and thenano-particles in the second group are arranged in the second region tooptimally detect electromagnetic radiation in a second wavelength rangedifferent from the first wavelength range.

According to another aspect of the embodiment, the nano-particles ineach pixel further comprise a third group of nano-particles arranged ina third region of the pixel, the nano-particles in the third group arearranged in the third region to optimally detect electromagneticradiation in a third wavelength range different from the firstwavelength range and the second wavelength range.

According to another aspect of the embodiment, the array is polarizationsensitive to the electromagnetic radiation in the predeterminedradiation range.

According to another aspect of the embodiment, the nano-particles of thefirst group are sensitive to a polarization of the electromagneticradiation in the predetermined radiation range in a first polarizationdirection, and the nano-particles of the second group are sensitive to apolarization of the electromagnetic radiation in the predeterminedradiation range in a second polarization direction.

According to another aspect of the embodiment, the predeterminedwavelength range comprises multiple predetermined wavelength subranges.

According to another aspect of the embodiment, the predeterminedwavelength subranges are separated from each other.

According to another aspect of the embodiment, the predeterminedwavelength subranges are continuous with each other.

According to another aspect of the embodiment, the predeterminedwavelength range is between the deep ultra-violet and microwave regionsof the electromagnetic spectrum.

According to another aspect of the embodiment, the predeterminedwavelength range comprises at least one of the visible, near infra-red,short-wave infrared, medium-wave infrared, infra-red, ong-waveinfra-red, and tera-hertz regions of the electromagnetic spectrum.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 a is a diagram illustrating the energy levels atmetal-semiconductor interface.

FIG. 1 b is a diagram illustrating the energy levels atmetal-semiconductor interface with a plasmon excitation.

FIG. 1 c is a diagram illustrating the energy levels atmetal-semiconductor interface under (a) equilibrium, (b) forward biasV_(f), and (c) reverse bias V_(r) conditions.

FIG. 2 is a graph showing the barrier resistance versus the bias voltagefor different metal-semiconductor interfaces.

FIG. 3 is a graph showing the thermionic current versus applied biasvoltage for a single gold nanorod of length 1 micron and diameter 0.1microns in Si<100>, n-doped at 10¹⁵ cm⁻³.

FIG. 4 is a graph showing the forward-bias cutoff frequency versus thedopant concentration.

FIG. 5 is a schematic of an electromagnetic radiation detector accordingto an embodiment of the invention.

FIG. 6 is a schematic of an electromagnetic radiation detector accordingto another embodiment of the invention.

FIG. 7 illustrates a top view of a portion of the detector material ofthe detector of FIG. 6.

FIG. 8 is a schematic showing a post-detection signal-processing circuitfor ultra-low current detection with minimal noise to be used with thedetector according to an embodiment of the invention.

FIG. 9 is a graph showing the absorption cross section versus cylinderlength for 10 nm diameter gold nanorods embedded in air for 8 and 12micron EMR wavelengths.

FIG. 10 is a graph showing the absorption, cross section versus cylinderlength for 100 nm diameter gold nanorods embedded in air for 8 and 12micron EMR wavelengths.

FIG. 11 is a graph showing the absorption cross section versusnanosphere diameter for gold nanospheres embedded in air for 8 and 12micron EMR wavelengths.

FIG. 12 is a graph showing the absorption cross section versus cylinderlength for 10 nm diameter gold nanorods embedded in a material with adielectric constant of 3.5 for 8 and 12 micron EMR wavelengths.

FIG. 13 is a graph showing the absorption cross section versusnanosphere diameter for gold nanospheres embedded in a material with adielectric constant of 3.5 for 8 and 12 micron EMR wavelengths.

FIG. 14 illustrates the structure of a two-dimensional infinite arrayand the definition of a coordinate system with P as an observationpoint.

FIGS. 15 a, 15 b and 15 c respectively illustrate of a detector havingdifferent polarization geometries showing nanoparticles array withrespectively a checker-board geometry, a parallel array geometry, anddifferent parallel array geometries placed in different quadrants in asingle pixel.

FIG. 15 d is a graph illustrating the absorption for a single nanorodversus lattice spacing of a nanorod array as a function of nanorodspacing.

FIG. 16 is a graph illustrating the responsivity as a function ofnanorod length for 10 nm diameter nanorods embedded in air, for 8 and 12micron EMR wavelengths.

FIG. 17 is a graph illustrating the responsivity as a function ofnanosphere diameter for gold nanospheres embedded in air for 8 and 12micron EMR wavelengths.

FIG. 18 is a graph illustrating the responsivity as a function ofnanorod length for 10 nm diameter nanorods embedded in air, for 8 and 12micron EMR wavelengths, and compared to a microbolometer responsivity.

FIG. 19 is a graph illustrating the responsivity as a function ofnanosphere diameter for gold nanospheres embedded in air for 8 and 12micron EMR wavelengths, and compared to a microbolometer responsivity.

FIG. 20 is a graph illustrating the responsivity as a function ofnanorod length for 20 nm diameter gold nanorods in a checker-boardpattern embedded in silicon, for 8 and 12 micron EMR wavelengths, andcompared to a microbolometer responsivity.

FIG. 21 is a graph illustrating the absorption for a single nanorodversus lattice spacing of a nanorod parallel array as a function ofnanorod spacing, for 20 nm diameter gold nanorods embedded in gold.

FIG. 22 is a graph illustrating the responsivity as a function ofnanorod length for 20 nm diameter gold nanorods in a parallel arraypattern embedded in silicon, for 8 and 12 micron EMR wavelengths, andcompared to a microbolometer responsivity.

FIG. 23 is a graph illustrating a system MTF as a function of spatialfrequency.

FIG. 24 is a graph illustrating a MRTF as a function of spatialfrequency using gold nanorods of 10 nm diameter.

FIG. 25 is a graph illustrating the MRTD as a function of spatialfrequency for various LWIR detectors.

FIG. 26 is a graph illustrating the figure of merit, D* as a function ofwavelength for 10 nm diameter gold nanorods embedded in silicon.

FIG. 27 is a schematic of an electromagnetic radiation detectoraccording to another embodiment of the invention, where nanoparticlesare embedded in a heterojunction.

FIG. 28 is a schematic of an electromagnetic radiation detectoraccording to another embodiment of the invention, with an orthogonalimplementation for current generation.

FIG. 29 is a schematic of an electromagnetic radiation detectoraccording to another embodiment of the invention, with semiconductornanoparticles embedded in a metal matrix.

FIG. 30 is a schematic of an electromagnetic radiation detectoraccording to another embodiment of the invention, with an avalanchephotodiode structure.

FIG. 31 is a graph showing the absorption cross section versus cylinderlength for 10 nm diameter gold nanorods for 1, 2 and 3 micron EMRwavelengths.

FIG. 32 is a graph showing the absorption cross section versus incidentEMR wavelength for 10 nm, diameter, 400 nm length gold nanorods.

FIG. 33 is a graph showing SWIR-APD gained responsivity as a function ofnanorod length for 6600 gold nanorods in silicon for EMR differentwavelengths, and as compared to InGaAs diodes a 2 and 2.65 microns.

FIG. 34 is a graph showing the enhanced absorption versus latticespacing for the nanorod system of FIG. 33 for a nanorod spacing of 150nm.

FIG. 35 is a graph showing the figure of merit, D* as a function ofwavelength for a SWIR detector.

FIG. 36 is a graph showing the absorption cross-section as a function ofnanorod length for 22 nm diameter gold nanorods in silicon, at EMRwavelengths of 450 nm, 500 nm, 615 nm, 700 nm, 800 nm and 1-micron.

FIG. 37 is a graph showing the responsivity for ˜5000 gold nanorods insilicon as a function of nanorod length.

FIG. 38 a is a graph showing the complex refractive index curves ofsilicon carbide as a function of EMR wavelength in the LWIR region.

FIG. 38 b is a graph showing the polarizability of 10 nm diametersilicon carbide spheres as a function of EMR wavelength in the LWIRregion.

FIG. 39 is a graph showing the aborption as a function of EMR wavelengthfor 20, 40, 100, 200 and 400 nm diameter silicon carbide spheres in theLWIR region.

FIG. 40 is a schematic illustrating of an enhanced photodetection systemusing gold nanoparticles.

FIG. 41 is a schematic illustrating arrays of differently oriented andlength nanorods for wavelength and polarization selectivity in a singlepixel detector.

FIG. 42 are graphs of the responsivity versus wavelength showing therelative spectral bandwidths that may be obtained in two spectralregions using the arrays of FIG. 35 for simultaneous SWIR and LWIRdetection.

DETAILED DESCRIPTION Introduction

The references cited herein are incorporated by reference in theirentirety.

The present inventors have found that by appropriately selecting thegeometry and materials for a detector material comprising asubstantially regular array of nanoparticles embedded in a matrixmaterial, and by applying an appropriate bias voltage to the matrixmaterial, a direct electrical current may be generated based on acooperative plasmon effect in the detector material when EMR in adesired wavelength range is incident upon the detector material, andwhere the dominant mechanism for decay in the cooperative plasmon effectis nonradiative. The direct electronic mechanism used for detection hasadvantages over other current detection systems for EMR detection.

A primary focus is the use of plasmonic effect maximized absorption bynanoparticles, such as metal nanoparticles, embedded in a matrixmaterial, such as a semiconductor material, for the purpose of detectingincident EMR. This mechanism is precisely the opposite effect which hasbeen used in many optical nanoparticle systems, where the employment ofa plasmonic effect is used to maximally scatter incident EMR into thesurrounding semiconducting matrix material to enhance the EMR absorptionin that material.

The selection of the geometry and materials to maximize the absorptionmay involve (1) the choice of nanoparticle material and matrix material,(2) the size and shape of nanoparticles, (3) the arrangement of suchnanoparticles in different kinds of substantially regular arrays, and(4) an appropriately applied bias voltage, so as to generate both (A) ahigh electric field in the vicinity of the nanoparticles and (B)subsequent electric current to flow, which can provide a measurement ofthe fluctuations in the strength of the incident EMR and itsfluctuations. Thus the fluctuations of the incident EMR can be sensed,either photovoltaically or photoconductively, according to peripheralcircuitry into which this nano-plasmonic (meta-material) EMR-sensor isplaced.

Further parameters for providing an appropriate EMR detector mayinclude, such as in the case where the matrix material is asemiconductor, for example: (5) varying the doping level of thesemiconductor material adjacent to the nanoparticle arrays, and (6) theuse of multiple semiconductor materials to form hetero-junctions inwhich the nanoparticle arrays are embedded.

The selection of the geometry and materials will in general depend uponthe wavelength of the EMR desired to be detected. In general, thedetector may be tailored for EMR detection desired regions across theEMR spectrum such as from the deep ultra-violet to the microwave region,through both the visible (VIS) and all infrared (IR) regions, such asnear IR (NIR), long wavelength (LWIR), mid IR (MWIR), and shortwavelength IR (SWIR).

For example, for detectors in the LWIR region (approximately 8 to 14microns wavelength), gold nanorods may be embedded in a silicon matrix,and provide a number of advantageous features. Such LWIR detectors mayprovide enhancements of responsivity/sensitivity as compared tomicro-bolometers at LWIR wavelengths. Furthermore, arrays of nanorodsmay arranged to be polarization sensitive, creating a new sensingparameter, whereas the symmetry of spheres as nanoparticles may notallow for such polarization sensitivity. Further, detector frequencyresponse is good, as plasmon decay times are of order of less than 1picosecond and 10 micron pixel electrical time constants are around 50micro-seconds (20 KHz).

Such nanoparticle array detectors using gold nanorods may provideadvantages over microbolometer detectors. In particular, suchnanoparticle array detectors may outperform correspondingmicro-bolometers by substantially more than an order of magnitude, andthus represent a cost-effective, competitive and worthwhiledetector-performance improvement leading towards higher resolution andless blurred moving images, together with much greater temperaturesensitivity and gray scale within images.

For example, for a f/1, 10 micron square pixel, closepacked gold nanorodarray system, preliminary estimates of Noise Equivalent TemperatureDifference (NETD) are in the sub milliKelvin (mK) region, MinimumResolvable Temperature Difference (MRTD) being mostly sub-mK across theentire Modulation Transfer Function (MTF). 120 Hz and greater frame ratedisplays are thus made possible for the LWIR region, providingapplications for night vision. These numbers compare favorably tomicrobolometers in the LWIR because of the direct electronic mechanismused for detection rather than the usual thermal fluctuation mechanismof micro-bolometers.

BACKGROUND TECHNOLOGY FOR NANOPARTICLES AND NANOPARTICLE ARRAYS (i)Polaritons and Plasmons

As technological background for the present invention, an importantaspect is electronic states called polaritons, which are surfaceplasmons. Surface plasmons and their properties are described in manytexts. An excellent review is given by [Zayats, A. V., Smolyaninov, I.I. and Maradudin, A. A., 2005; ‘Nano-optics of surface plasmonpolaritons’, Physics Reports 408, pp 131-314]. The essential math forcalculation of surface plasmon characteristics is contained therein.Surface plasmons are associated with surfaces, such as substrates. Incontrast to surface plasmons, localized plasmons are plasmons associatedwith small particles of size less than the incident wavelength, i.e.,<λ.

For a localized plasmon excited by photon absorption, there are twocompeting processes for decay, a radiative decay process into photons,dominating for larger particles, and a nonradiative process due toabsorption. For this invention, it is desired to maximize thenonradiative process for the purposes of detection, which also minimizesthe radiative decay process. The nonradiative decay is due to thecreation of electron-hole pairs via either intra-band excitations withinthe conduction band, or inter-band excitations from the lower lying dbands to the (surface plasmon) conduction band (in noble metals such asgold). Radiation damping in localized plasmons is caused by directradiative decay of the coherent electron oscillation into photons. Suchradiative decay is not the desired decay mechanism for the detectorsdescribed herein.

By using a model for both these possibilities [Zayats, A. V.,Smolyaninov, I. I. and Maradudin, A. A., 2005; ‘Nano-optics of surfaceplasmon polaritons’, Physics Reports 408, pp 131-314], i.e., radiativeand non-radiative decay, a ‘quantum efficiency’ for the light scatteringaspects (due to radiative decay) may be estimated. For the detectors ofthe present invention it is desired to minimize the light scatteringefficiency so that absorption and thus the responsivity is maximized.

(ii) Metal Nanoparticles

Single Nanorods

Computation of the optical and absorption properties of singlearbitrarily shaped nanorod nanoparticles proceeds from knowledge of theoptical constants of the nanorod material across the infrared spectrumfrom the near and short-wave end to the long-wave end, around 13 micronswavelength. For example, such data is found for gold in [Palik, E. D.,1985; ‘Handbook of Optical Constants of Solids’, Academic Press].

To determine the absorption cross-section of the particle under study,Cabs, the complex refractive index n+ik may be used, and the complexdielectric function (or relative permittivity) ∈=∈′+i∈”; where ∈′=n²−k²and ∈″=2 nk. From the relationship of the complex dielectric function asa function of wavelength (λ), the polarizability (α) of any shape ofparticle may be calculated using various equations for different shapesof particle, [van de Hulst, H. C., 1957 & 1981; ‘Light Scattering bySmall Particles’, Dover].

Once the polarizability α is determined, the absorption cross-section ofthe particularly shaped particle under study, Cabs, may be calculatedbased on its shape and material, along with its related quantity, theabsorption efficiency, Qabs, [van de Hulst, H. C., 1957 & 1981; ‘LightScattering by Small Particles’, Dover]. Both of these parameters maythen be used in the estimation of responsivity and other sensitivityparameters. The aspect ratio and size of a nanorod directly affects thenanorod's polarizability, α, and may be approximated by variousdifferent means [van de Hulst, H. C., 1957 & 1981; ‘Light Scattering bySmall Particles’, Dover].

From the Cabs, the absolute responsivity using incident illuminationpower, the coupling constant to the detector's electrical circuit, andpolarization conditions may be calculated, as will be described below.Selecting and optimizing the plasmon decay mechanism, and computing thedetector's substrate coupling constant are both of importance. Withnanorods, localized plasmons, not surface plasmons, are the plasmons ofinterest. As described below, localized plasmons may be used to excitesurface plasmons in the detector substrate material providing anefficient power transfer into a detector geometry.

Clusters of Nanorods

Substantially regular arrays of nanoparticles are employed in thedetectors described to provide an appropriate electronic decay mechanismfor absorption. Thus, it is important to look at clusters ofnanoparticles, such as nanorods, in determining the decay mechanism. Theregularity of the arrays and the spacing between particles areparameters in minimizing radiative decay.

Clusters of nanorods cause changes to the plasmon resonance conditions,depending strongly on the separation of the nanorods compared to theircharacteristic dimensions. Such effects are known, such as described in[Kreibig, U., & Vollmer, M., 1995; ‘Optical Properties of MetalClusters’, Springer]; although exact calculation may be difficult.

Two regimes are distinguished depending on the magnitude of theinter-particle spacing, d. Using closely spaced particles, d<<λ (thewavelength of the incident EMR), near-field interactions with adependence on d⁻³ dominate. Light scattering is strongly reduced forclosely spaced particles, and thus such closely spaced particles areuseful in the detectors as described herein. Interparticle couplingleads to shifts in the spectral position of plasmon resonance. Withrods, there is a blue-shift (toward higher energy) for the excitation oftransverse modes, and a red-shift (toward higher energy) forlongitudinal modes. For larger particle separations, far field dipolecoupling with d⁻¹ dependence dominates.

Nanorods on Surfaces and Coupling

Interactions between nanorods can be enhanced by providing additionalcoupling pathways, for example, seeding surface plasmons of the detectormaterial surface from localized plasmons in the nanorods by use of aconducting substrate. This is useful to maximize coupling into adetector.

There is experimental evidence of changes in plasmon resonanceconditions of arrays of nanoparticles on surfaces, some describing thecoupling of arrays of nanoparticles to surface plasmons in the substratematerial. Further, quantification of energy coupling effects of arraysof nanorods is described in [Yamaguchi, T., et al, 1974; ‘Optical effectof the substrate on the anomalous absorption of aggregated silverfilms’, Thin Solid Films, 21, pp 173-187], and a mathematical discussionin great detail of the induced dipole-strengths of a particle on asurface is described in [Mills, D. L., 2002; ‘Theory of STM-inducedenhancement of dynamic dipole moments on crystal surfaces’, PhysicalReview B, 65, page 125419]. These theories are useful in the computationof detector responsivity and for the calculation of energy couplingconstants for various different media surrounding gold nanorod arrays ona surface, and for various different media forming the substratematerial.

A valuable method for computing the polarizability Pj of an individualparticle in an array has been described by [Wokaun, A., 1985; ‘Surfaceenhancements of optical fields’, Molecular Physics, 56, 1, pp 1-33] asfollows:

${P_{j} = {\frac{1}{4\pi}\frac{\left( {ɛ - 1} \right)\left( {1 - {q^{2}/10}} \right)}{1 + {\left( {ɛ - 1} \right)A_{{eff}.{jj}}}}E_{0j}}},{j = x},y,{z.}$

where A_(eff) is the effective shape dependent polarization constant ofan individual particle and q=k(abc)^(1/3) where a, b, c are theparticle's half-axis lengths, and k is the scattering vector.

EMR Scattering and Absorption

Gans' mathematics for calculation of light-scattering and absorptioncross-section based on particle-shapes is described in [Link, S. andEI-Sayed, M. A., 2000; ‘Shape and size dependence of radiative,nonradiative and photothermal properties of gold nanocrystals’, Int.Reviews in Physical Chemistry, 19, 3, pp 409-453]. The opticalabsorption spectrum of a collection of randomly oriented gold nanorodswith aspect ratio R can be modeled using an extension of the Mie theory.Within the dipole approximation according to the Gans treatment, theextinction cross-section σ_(ext) for elongated ellipsoids is given bythe following equation:

$\sigma_{ext} = {\frac{\omega}{3c}ɛ_{m}^{3/2}V{\sum\limits_{j}\frac{\left( {1/P_{j}^{2}} \right)ɛ_{2}}{\left\{ {ɛ_{1} + {\left\lbrack {\left( {1 - P_{j}} \right)/P_{j}} \right\rbrack ɛ_{m}}} \right\}^{2} + ɛ_{2}^{2}}}}$

where P_(j) are the depolarization factors along the three axes A, B andC of the nanorod with A>B=C, defined as:

${P_{A} = {\frac{1 - {\mathbb{e}}^{2}}{{\mathbb{e}}^{2}}\left\lbrack {{\frac{1}{2{\mathbb{e}}}{\ln\left( \frac{1 + {\mathbb{e}}}{1 - {\mathbb{e}}} \right)}} - 1} \right\rbrack}},{P_{B} = {P_{c} = \frac{1 - P_{A}}{2}}},$

and the aspect ratio R is included in e as follows:

$e = {\left\lbrack {1 - \left( \frac{B}{A} \right)^{2}} \right\rbrack^{1/2} = \left( {1 - \frac{1}{R^{2}}} \right)^{1/2}}$

Semiconductor Physics and Optoelectronics Effect of Metal-SemiconductorInterfaces

Metal-semiconductor interfaces have long been used in photodetection andsolar-cells. A modern treatment is given in [Chuang, S. L., 1995;‘Physics of Optoelectronic devices’, p 342, Wiley] and [Sze, S. M., &Ng, K. K., 2007; ‘Physics of Semiconductor Devices’, 3^(rd) Edition,Wiley-Interscience], which is the origin of FIGS. 1 a, 1 b & 1 c. When asemiconductor is in intimate contact with a metal, the Fermi levels ofboth materials line up, and a barrier is formed over (or through) whichcarriers must pass during current flow (see FIG. 1 a). In this case thecurrent is majority carriers, unlike a p-n junction where the minoritycarriers dominate.

For example, for gold metal nanorods in a semiconductor matrix, withplasmon excitation based on LWIR EMR incident, a “cloud of electrons(plasmon) is excited. A high electric field is created at the surface ofthe nanorods. Thus the majority carriers are electrons, in which casethe appropriate theory to be used here is a n-type doped semiconductor(see FIG. 1 b).

When a voltage bias is applied, the line up of the energy levels ischanged, per FIG. 1 c and the flow of electrons into and out of themetal is altered. The Schottky barrier is effectively lowered byapplying a forward bias. For smaller photon energy, hv, than thebandgap, Eg, (longer wavelength) qφB<hv<Eg the photoexcited electrons inthe metal can surmount the barrier and be collected by the semiconductor(see FIG. 1 b).

While silicon could be used as the semiconductor matrix material, othersemiconductor materials could also be used. For example, GaAs could beused for higher speed detectors or to exploit different barrierproperties. One advantage of using silicon is that it is already highlydeveloped in volume production of CMOS detector arrays comprising tensof millions of ultra-low noise (rms ˜2e−) pixels with associatedread-out circuitry.

The nanorod array designs may be retrofit into standard Si-CMOSdetector-arrays, using a 10 micron pixel capability instead of thetypical 1-2 micron pixel capability used in many camera detector arrays.This may facilitate rapid transition to high volume manufacturing whileallowing for using multi-mega-pixel imaging.

In an n-type arrangement under bias, the current is dominated byelectron-flow from the metal into the semiconductor, and the totalthermionic current is given by the following equation [Sze, S. M., & Ng,K. K., 2007; ‘Physics of Semiconductor Devices’, 3^(rd) Edition,Wiley-Interscience]:

$I:={A \cdot R \cdot \Gamma^{2} \cdot {\exp\left( \frac{{- \phi} \cdot q}{k \cdot \Gamma} \right)} \cdot {\exp\left\lbrack {\left( \frac{V \cdot q}{k \cdot \Gamma} \right) - 1} \right\rbrack}}$

where V is the applied bias voltage V, φ the barrier height, T is thetemperature (deg K), k is Boltzmann's constant, q is the electroncharge, R is the Richardson constant, and A is the cross-sectional areaof the metal.

For Si<100> and Si<111>, the values of R are known to be ˜246 and ˜258respectively. For GaAs, R ˜8.04.

The depletion region width of the semiconductor, W, is estimated from:

$\text{W:} = \left\lbrack {2 \cdot \frac{ɛ\;{s \cdot \left( {{Vbi} - V} \right)}}{q \cdot {Nd}}} \right\rbrack^{\frac{1}{2}}$

where ∈s is the relative permittivity of the semiconductor, Nd is thedopant concentration, and Vbi is the built-in potential due to themetal-semiconductor interface, which for gold-silicon is −0.297 Voltsfor 10¹⁵ cm⁻³ n-type doping. For sub-Volt detector bias, the depletionregion will extend to ˜1 μm to 3 μm.

For the barrier to work effectively, the doping of the semiconductor ispreferably light to medium in level, somewhere between about 10¹⁵ cm⁻³and 5×10¹⁶ cm⁻³. Under high doping conditions of, say, ˜10 ²⁰ cm⁻³,tunneling through the barrier is encountered. Such high dopingconditions could be a basis of a variation on a gold nanoparticlesilicon matrix detector design. The doping may be performed, forexample, using arsenic doping, such as by arsene gas by diffusion at1200° C., or by ion-implantation, for example.

The doping and the metal-semiconductor interface affect the barrier. Thebarrier resistance versus applied bias for different contacts to siliconis shown in FIG. 2. From FIG. 2 it can be seen that a bias voltage ofaround ˜0.5 Volts is useful.

The dark current also depends on the metal-semiconductor interface anddoping. For an exemplary single gold nanorod of 1 micron length and 0.1microns diameter embedded in Si<100>, n-doped at 10¹⁵ cm⁻³ thethermionic current can be calculated as shown in FIG. 3.

The frequency response of the detector based on the metal-semiconductorinterface may also be estimated. In FIG. 4 the frequency responses ofvarious metal-Si visible light photodetectors whose pixel sizes are1.0-40.0 microns is shown. Based on this, it is expected that theintrinsic performance will be in the tens of GHz range for a detectordesign using nanorods, when the n doping is at ˜10¹⁶ cm⁻³.

EMBODIMENTS

FIG. 5 illustrates a schematic of an EMR detector according to oneembodiment of the invention. The EMR detector includes a detectormaterial 216 and a voltage biasing element 240. The detector material216 includes a substantially regular array 212 of nanoparticles 210embedded in a matrix material 214. The voltage biasing element 240 isconfigured to apply a bias voltage to the matrix material 214 such thatelectrical current is directly generated based on a cooperative plasmoneffect in the detector material when electro-magnetic radiation in apredetermined wavelength range is incident upon the detector material,where the dominant mechanism for decay in the cooperative plasmon effectis non-radiative.

LWIR Detector

FIG. 6 illustrates a LWIR detector according to an embodiment of theinvention. The detector in FIG. 6 includes a detector material 316 on asubstrate 328, a top electrode 324, and a bottom electrode 326, wherethe top electrode and bottom electrode function as the voltage biasingelement. The detector material 316 comprises a substantially regulararray 312 of nanoparticles 310 embedded in a matrix material 314.

In this embodiment the nanoparticles 310 may be gold nanorods, which areembedded in the matrix material 314 which may be n-doped silicon. Then-doped silicon may have a doping concentration between about 10¹⁵ cm⁻³and 10²⁰ cm⁻³, for example.

FIG. 7 illustrates a top view of a portion of the detector material 316showing one pixel. As shown in FIG. 6, laterally surrounding thedetector material 310 is a surface passivation layer 322, which may bean oxide layer about ≧0.1 micron thick. The substrate 328 may have anintermediate layer 318, which may comprise n-doped silicon with a dopingconcentration similar to that of the matrix material 314, and which mayhave a thickness, for example, of about two microns. The substrate 328may also have a lower layer 320, which may be a n⁻ or n⁺ doped <100> or<111> silicon layer for example.

The detector may also have a guard-ring structure (not shown) to improvedevice performance as desired. The semiconductor materials of thedetector may also have graded dopant-levels, as desired, to improve theperformance.

The bottom and top electrodes 326 and 324 may be Ohmic contacts innature, and may be formed of a suitable conductor such as Pt—Si or Al,for example, and may have a suitable high doping in the semiconductormaterial immediately adjacent to those metals. The top electrode shouldbe sufficiently thin, such as approximately 10 nm thick or less, so thatit is a transparent conductor to the desired EMR to be detected.

The dominant dark current in this embodiment is thermionic emission ofthe majority carriers, the electrons; but for high doping of the matrixmaterial 314, tunneling transmission of electrons through themetal-semiconductor barrier provides a significant contribution to thecurrent, and thus also to the dark current. The signal current isprimarily the electron-emission over the barrier as shown in FIG. 1 b,created by the applied bias voltage, but the tunneling-current can alsoprovide a significant enhancement.

The detector of FIG. 6 may be formed by known semiconductor processingtechniques. An example follows. The bottom electrode 326 may be formedon the lower layer 320 of the substrate 328, where the lower layer 320may be n or n+Si<100> or Si<111>. The upper layer 318 may be formed, forexample, as a thin layer of n-doped Si, about 1 micron thick, dopedsomewhere between about 10¹⁵ cm⁻³ and 5×10¹⁶ cm⁻³, for example. A mask,such as a photolithographic mask, may be formed on the upper layer 318to define pixels of a size 10 microns square, where each hole in themask material defines the pixel size for a respective pixel. Thenanoparticles 310, such as gold nanorods, may then be formed within thehole in the mask, with a regular spacing between the nanoparticles 310.The matrix material 314 may be formed on the nanoparticles 310, bycoating the nanoparticles 310 with amorphous Si, for example, to a depthof about 100 nm, where the Si is doped somewhere between about 10¹⁵ cm⁻³and 5×10¹⁶ cm⁻³. The mask material may then be removed. The matrixmaterial 314 may be coated with about ≦10 nm thickness of Pt—Si tocreate the top electrode 324 as an ohmic contact that is transparent toLWIR. The 10 micron pixel area may be mask blocked and a thin (0.1 μm to1.0 μm) layer of SiO₂ may be formed over the area surrounding the pixel,and down the sides of the device, to act as the surface passivationlayer 322 and to block current-leakage. Wire bond contacts to the topand bottom electrodes 324 and 326 may be then be formed to allow for anapplication of bias voltage.

As an alternative to defining the pixel by forming a mask on the upperlayer 318, a SiO₂ overcoat may be formed on the upper layer 318 to actas the passivation layer 322, and the passivation layer 322 thenpatterned to define the pixel regions. In this process a photoresist maybe formed on the passivation layer 322, and the photoresist may beilluminated through a photomask, and then developed to define the pixelregions. The passivation layer 322 may then be etched down to the upperlayer 318 using the patterned photoresist as an etch mask. Amorphous Simay then be deposited in the pixel regions to a depth of 1-2 microns andn-doped to about between 10¹⁵ cm⁻³ and 5×10¹⁶ cm⁻³, followed by applyinga regular array of nanoparticles 310, such as gold nanorods, to theamorphous Si. Si is then deposited to a thickness of about 0.1 micronsto embed the nanoparticles 310, and form the matrix material 314 alongwith the deposited amorphous silicon. The matrix material 314 may becoated with about <10 nm thickness of Pt—Si to create the top electrode324 as an ohmic contact that is transparent to LWIR. A suitable area ofSiO₂ passivation is applied around the edges of the pixel to finish thepassivation layer 322. Wire bond contacts to the top and bottomelectrodes 324 and 326 may be then be formed to allow for an applicationof bias voltage. Optionally, a guard ring could be formed.

Electronics for Low-Noise Post-Detection Processing for the LWIRDetector

A circuit for low-noise post-detection processing for the LWIR detectormay be provided as shown in FIG. 8. The circuit may have a verylow-noise pre-amplifier, such as for example, the AMPTEK A225 (or A250)followed by analog-threshold discrimination, the A206 voltage amplifierand low-level discriminator. Use of groundplane shielding on the circuitcard, and possibly Faraday cage isolation to reduce pick-up may also beused.

Nano-Particle Parameters and Device Performance

To determine the device performance, we presume a pixel with a goldnanorod array arranged as shown in FIG. 7. Rows of precisely-spacednanorods are provided to enhance the plasmon response to the detectedEMR. The pixel size is taken to be a 10 micron square detector element,such as might be typical for use with LWIR. In practice, the size couldbe reduced to a 1 micron square for the purpose of super resolutionimaging.

We further assume 400 nm long, 20 nm diameter gold nanorods arranged ina parallel array lattice, spaced for a maximum polarizability responsewhere the nanorods are separated by an optimum 0.3 microns. In thiscase, we have 33 nanorods in each row, where we further presume thesenanorods are embedded in silicon of <0.1 micron thickness. The number ofrows of these nanorods in the pixel is 25 to provide adequate row-to-rowseparation for row independence. The pixel thus has ˜830 gold nanorodsin an array contributing to separation enhanced polarizability, and thusto the signal strength. For the arrangement of FIG. 7, where the longaxes of the nanorods are aligned in the same direction, the array candetect a chosen single polarization (oriented along the direction of thelong axes), and thus is polarization selective.

The detector performance depends on the detector structure, and inparticular on the parameters of nanorod shaped nanoparticles, withdifferent aspect ratios, surrounding medium, polarization of theincident EMR, and regular arrays of particles, as described below.

Different Aspect Ratios for Single Nanoparticles

The results shown in FIG. 9 illustrate results of calculationsdetermining if any resonance or other optimized condition exist for goldnanorods in the wavelength range of interest, in this case LWIR.

Using the mathematics described in the Technology Background sectionabove, the FIG. 9 curves are calculated for the absorption crosssections of 8 and 12 micron LWIR radiation for 10 nm diameter goldnanorods, as a function of nanorod length, up to the point where thescattering/absorption approximations become invalid (approximately1/10th wavelength, i.e, a 1 micron rod length). As shown, the peakabsorption cross section is in the region of nanorod length 600 nm to900 nm.

To see the magnitude of the effect of aspect ratio on the absorptioncross section, the absorption cross sections for 100 nm diameternanorods were computed as shown in FIG. 10. In this case, the peakabsorption cross section region has moved out to ˜1500 nm-2000 nm forthe LWIR wavelengths, and the absorption cross section has increased by2 orders of magnitude.

The absorption cross sections for 1000 nm diameter nanorods werecomputed as shown in FIG. 11. For FIG. 11, the diameter of the rods werefixed at 1000 nm, while the cylinder length was increased up to 1000 nmand just beyond to the peak, so the nanoparticle goes from a disk to asphere as the cylinder length increases to 1000 nm, and peaks as anoblate sphere shape at ˜1500 nm.

From FIGS. 9-11 it can be seen that the LWIR absorption cross section iscalculated to be yet another two orders of magnitude higher, and thatcondition occurs when the nanorod has become a slightly oblatenanosphere at around 1500 nm-2000 nm (2 microns) diameter for the LWIRspectral region. It should be noted that for the parameters in FIGS.9-11, the theory for calculation is at the edge of its validity. Forparticles embedded in media other than air, as described below, thevalidity improves satisfactorily.

Nanoparticles Surrounded in Different Mediums

FIGS. 9-11 examine the absorption characteristics of gold nanorodsembedded in air, such as might occur on top of a detector substrate. Inthis section, there is described the absorption characteristics for goldnanorods embedded in media other than air.

If FIG. 9 is recalculated using a dielectric constant value of 3.5,greater the 1.0 value for air, for the surrounding medium we get FIG.12. As shown in FIG. 12, the absorption cross section of the nanorodshas reduced in value, as compared to. FIG. 9, by a factor ˜×10, i.e.,one order of magnitude. The peak absorption cross section has also beenreduced to nanorod lengths of 300 microns to 500 microns. Thecorresponding recalculation of FIG. 11, where the nanoparticles are nowembedded in a dielectric constant of ˜3.5, which peaks for near spheres,is shown in FIG. 13.

Not only is a reduction observed in the absorption cross section ofnearly an order of magnitude (as with the nanorod case in FIGS. 12 and9), but the optimum sphere size can be seen for the absorption of 8-12micron LWIR to also be reduced—to become a nanosphere diameter ofapproximately 1 micron in this dielectric material.

If the dielectric constant is increased to 7.0, the sphere's peakabsorption cross section range reduces further, to 600-900 nm diameter,and the absorption cross section decreases by another half-order ofmagnitude.

Different Polarizations

In all the above calculations for different aspect ratios and differentsurrounding media, the polarization of the incident LWIR is parallel tothe long axis of the gold nanorods.

If we examine the orthogonal polarization, the transverse plasmon isexcited instead of the longitudinal plasmon. In this case, theabsorption coefficients are correspondingly smaller by roughly theaspect ratio. Thus in deploying any nanorods for unpolarized LWIRdetection some regular (or random) array of rod axis directions would benecessary. For detection of polarized LWIR, orientation of the nanorodaxes (along the direction of the desired polarization) might be usedeffectively to discriminate in favor of detection of the desiredpolarization, and could reduce unwanted noise. For example, a 10 nmdiameter rod whose length is 1000 nm offers polarization selection100:1.

Regular Arrays of Nanoparticles

[Zhao, L. L., et al, 2003; ‘The Extinction Spectra of SilverNanoparticle Arrays: Influence of Array Structure on Plasmon ResonanceWavelength and Width’, J. Phys. Chem. B, 107, pp 7343-7350] and [Wokaun,A., 1985; ‘Surface enhancements of optical fields’, Molecular Physics,56, 1, pp 1-33] provide a theoretical basis for proceeding with aregular lattice of nanoparticles and computing the absorption crosssection characteristics. While the full mathematics becomes difficult;the simplest and most useful way to use the math in [Zhao, L. L., et al,2003; ‘The Extinction Spectra of Silver Nanoparticle Arrays: Influenceof Array Structure on Plasmon Resonance Wavelength and Width’, J. Phys.Chem. B, 107, pp 7343-7350] is as follows, where the equations relate tothe co-ordinate system shown in FIG. 14.

$\begin{matrix}{\alpha_{cluster} = {{\frac{\alpha_{s}}{1 - {\alpha_{s}S}}\mspace{14mu}{and}\mspace{14mu} C_{ext}} = {4\;\pi\;{{k{Im}}\left( \alpha_{cluster} \right)}}}} & (12)\end{matrix}$

S being the retarded dipole sum

$\begin{matrix}{S = {\sum\limits_{j \neq i}\left\lbrack {\frac{\left( {1 - {ikr}_{ij}} \right)\left( {{3\cos^{2}\theta_{ij}} - 1} \right){\mathbb{e}}^{{ikr}_{ij}}}{r_{ij}^{3}} + \frac{k^{2}\sin^{2}\theta_{ij}{\mathbb{e}}^{{ikr}_{ij}}}{r_{ij}}} \right\rbrack}} & (13)\end{matrix}$

where r_(ij) is the vector between the i-th and j-th nano-particles, andα_(s) is the polarizability for a single nanoparticle.

Equation (12) above may be used, where α_(cluster) is used in ‘C_(abs)’in the place of ‘C_(ext)’, where: C_(abs)=k Im {α_(cluster)} and{k=2π/λ}, which is valid if the scattering is very small compared to theabsorption [van de Hulst, 1981].

[Yamaguchi, T., et al, 1974; ‘Optical effect of the substrate on theanomalous absorption of aggregated silver films’, Thin Solid Films, 21,pp 173-187] provides expressions for the terms in S in the equationabove, calculated numerically:

$\begin{matrix}{{\sum\limits_{j}\frac{1 - {3\cos^{2}\theta_{j}^{\prime}}}{r_{j}^{3}}} = {{- 0.716}\frac{\mathbb{d}_{w}}{2a}}} & (10) \\{{\sum\limits_{j}\frac{1}{r_{j}^{3}}} = {0.716\frac{\mathbb{d}_{w}}{a}}} & \;\end{matrix}$

where dw is the mean thickness of the layer, and a is thelattice-constant, which is small compared to λ.

Thus, the following nanoparticle geometries may be calculated fairlyaccurately for their polarization and wavelength discriminationpotential advantages. FIGS. 15 a, 15 b, and 15 c illustrate differentpolarization geometries.

The geometry of FIG. 15 a offers the possibility of detecting orthogonal‘pure’ polarizations within a single pixel by electrically connecting tothe rows and columns. FIG. 15 b offers the possibility of discriminatinga single polarization state in a pixel. FIG. 15 c offers the possibilityof detecting multiple specific wavelengths, by using rod lengthvariations, and specific polarizations within the same pixel.

When the nanorods are arranged in parallel arrays, per FIG. 15 b, thespacing between the rods critically determines the effectivepolarizability of an individual rod, so that the number of rods may thenbe summed to get the total effective polarizability of the array. As anexample, consider the variation of polarizability of 400 nm long goldnanorods of 20 nm diameter illuminated by 10 micron wavelength LWIR. Theoptimum spacing of the rods is seen in FIG. 15 d to be around 10⁻⁷ m formaximum polarizability (signal response).

Responsivity

Calculation of the responsivity will depend on the mechanism used totransfer power from the nanoparticle array into an electrical signal.One mechanism that may used to transfer power from the nanoparticlearray into an electrical signal constituting the detection of incidentradiation, is by coupling the localized plasmon in the nanoparticlesinto the substrate for detection, i.e., using the electron clouddirectly. Such a mechanism is a most efficient way of using the‘electron-cloud’ localized plasmons induced in the nanoparticles by theincident LWIR to be detected.

Direct electrical monitoring of the Plasmon generation can be achievedin a number of ways, such as (1) resistance measurement, (2)potential/voltage-fluctuation measurement, or (3) capacitance-change.The direct electrical monitoring method chosen will depend on theparticular geometry in which the nanoparticle array is configured. Anelectric field may be applied via a dielectric interface, to enhance andmodify optical plasmon resonance.

As an example configuration used for the calculation, consider thefollowing structure. A regular square pattern array of gold nanorods of10 nm diameter and ˜1000 nm (1 micron) length, where the nanorods arearranged in a checkerboard array (see FIG. 15 a), on a silicon substratewhose pixels are 30 microns square. The square array comprises 30 rowsand 30 columns, thus yielding 900 nanorods on the pixel surface. Thisarrangement provides sufficient nanorod separation for an insignificantspectral shift of the longitudinal plasmon absorption peak, and allowsfor the detection of random polarized incident LWIR EMR (as the EMRdecomposes into the two orthogonal directions of the nanorods). For thecalculation an unpolarized LWIR of 8 and 12 microns wavelength ispresumed.

This array structure has an electron (dipole) coupling coefficient of˜0.001, because the nanorods on top of the substrate in air areprimarily reradiating photons. In this calculation the absorbed energy,not the scattered energy, is of interest.

The mean photon counting rate is inserted into the calculation using thevalue of the absorption cross section, Cabs, per FIG. 9. The detectorinternal quantum efficiency is conservatively presumed to be 50%. Thephotodetection rate may be converted to photoelectron current, using thefact that each photoelectron is 1.6*10⁻¹⁹ Amps. For calculationconvenience, the incident LWIR is assumed to be 1 Watt.

Using these parameters, the following responsivity curve as a functionof nanorod length is calculated, as shown in FIG. 16, where theresponsivity is calculated as Amps per Watt. FIG. 16 closely follows theshape of the FIG. 9, where the absorption cross section curve shownthere is converted into actual photodetection and then into coupledphotocurrent.

FIG. 17 illustrates the calculation for gold nanospheres on thesubstrate, for comparison. The peak in FIG. 17 occurs for ˜1.5-microndiameter nanospheres in air, so the lattice spacing should be increasedto about 6 microns, and the number of nanoparticles therefore falls tojust 25 over the pixel. The coupling coefficient is ˜0.01, because thephotons are not primarily reradiating as a nanorod typically does. Inspite of the reduced number of sphere shaped nanoparticles, theresponsivity has increased by about more than an order of magnitude atthe peak.

Comparison with Microbolometers

The responsivity of the system for FIGS. 16 and 17 can be compared witha microbolometer system. The principles and practice of microbolometerLWIR detection are well known and understood, for example as summarizedby [Kruse, P. W., 2001; ‘uncooled Thermal Imaging Arrays, Systems, andApplications’, SPIE Tutorial Texts in Optical Engineering, Vol. TT51]and [Radford, W. et al, 1999; ‘Sensitivity improvements in uncooledmicrobolometer FPAs’, Proc. SPIE, Vol. 3698, pp 119-130]. [Radford, W.et al, 1999; ‘Sensitivity improvements in uncooled microbolometer FPAs’,Proc. SPIE, Vol. 3698, pp 119-130] provides an example of amicrobolometer system with pixel dimensions of 50 microns, spectralperformance of 8-14 microns, absorptivity >80%, detector nonuniformityof 10%, and particularly the signal responsivity of >5×10⁶ Volts perWatt, which provides the basic sensitivity of the pixels. Using thestated nominal resistance of 50,000 Ohms, this translates to a basicminimum responsivity of 100 Amps per Watt at room temperature. Morerecent microbolometer systems perform even better than the Radfordspecifications.

The sensitivities of gold nanorod based detection and microbolometersmay now be compared for the same incident power in the 8-14 micron LWIRband, as shown in FIGS. 18 and 19. FIGS. 18 and 19 illustrate theresponsivity curves in FIGS. 16 and 17, but also include theresponsivity of a microbolometer system, the Raytheon Microbolometer[Radford, W. et al, 1999; ‘Sensitivity improvements in uncooledmicrobolometer FPAs’, Proc. SPIE, Vol. 3698, pp 119-130].

Based on the comparison shown, a nanosphere array in place of nanorodsmay provide advantages, at the expense of polarization discrimination.This is so even when some embedding matrix material is placed around anarray of micron sized gold spheres and thus the responsivity falls by anorder of magnitude or more. The nanosphere array may exceed themicro-bolometer performance by about a factor of 10 in practice.

Electrical and Thermal Time Constants

In this section, the thermal time constant of a microbolometer system iscompared with the electrical time constant of a system usingnanoparticles with direct electrical detection. Traditionally,microbolometers function through the absorption of LWIR as heat, andhave thermal response time constants of around a few milliseconds. Theheat detection mechanism of microbolometers provides a disadvantage formoving imagery, leading to often observed blurring of the images beingviewed. For the systems using nanoparticles, by comparison, where thedetection mode is based on direct electrical detection, the timeconstants may be much smaller, in the sub microsecond region.

The intrinsic thermal time-constant of a pixel is given by:

$\tau = \frac{p\; c_{p}V}{h\; A_{s}}$

where ρ is the density of the pixel material, c_(p) its specific heat, Vits volume, h its heat-transfer coefficient, and A_(s) its surface area.The above equation takes no account of any additional thermal loadsimposed by substrate support or wire connections, all of which need tobe made negligible by comparison.

The intrinsic electrical time constant of a pixel is RC, the product ofthe pixel's resistance R and its capacitance C, calculated from itsdimensions, resistivity and dielectric constant as is known. Again, thisequation takes no account of any additional resistive or capacitiveloads imposed by substrate support or wire connections, all of whichneed to be made negligible by comparison.

As examples, note that the thermal time constants for pure silicon andpure gold pixels of 10 microns square dimensions and 1 micron thickcalculate to be 10.95 milliseconds and 7.83 milliseconds, respectively.A composite structure employing silicon and gold lies somewhere betweenthese two values. Such a time constant is too slow for high resolution,blur free imaging of moving scenes.

By comparison, the electrical time constant of a silicon pixel, for5000Ω-cm Si, typically used for basic silicon structure manufacture, isonly 0.052 milliseconds, potentially ideal for blur free imaging withmoving scenes. In general, the resistivity of silicon depends on thedoping level. In our calculation here a resistivity of 10¹² cm⁻³ n-typeis used. Increased doping and the inclusion of gold nanorods would bothreduce the resistivity, and thus reduce further electrical timeconstant.

Structure for Detector Performance Calculations

The detector performance will of course depend upon the specificgeometry and materials. In this section the detector performance iscalculated for geometries of FIGS. 15 a and 15 b for a 10 microns squarepixel made of silicon embedded with 20 nm diameter gold nanorods, andwhere direct electrical detection of the induced plasmon cloud is usedfor signal creation. The further advantage of these two geometries istheir polarization sensing capability, which is unavailable to standardmicrobolometer detectors. The temperature of operation is presumed to beroom temperature.

Structure 1 For the geometry of FIG. 15 a, 400 nm long and 20 nmdiameter gold nanorods are used in the checkerboard lattice, with 25rows and columns. The nanorods are embedded in silicon of 1 micronthickness. The electrical time constant is ˜50 micro-seconds (˜20 KHzintrinsic response). There are 625 nanorods in the square lattice. Thisarray can detect orthogonal polarizations, and thus any polarizationstate. The polarizability of a single gold nanorod in silicon suggests a300 to 400 nanometer rod length is optimal, where the length is chosento be 400 nm. The 625 rod pixel's responsivity curves for 8 and 12micron wavelengths are shown in FIG. 20, which are very similar to thatof a microbolometer. The advantage for this structure is thepolarization detection, where any arbitrary polarization state can bediscriminated from the two orthogonal states detected in thechecker-board arrangement.

Structure 2 For the geometry of FIG. 15 b, 400nm long and 20 nm diametergold nanorods are used in the parallel array lattice. The nanorods arespaced for maximum polarizability response, which is a separation ofabout 0.3 microns, which provides for 33 nanorods in each row for a 10microns square pixel. The nanorods are embedded in silicon of 1 micronthickness. The electrical time constant for this structure is ˜50micro-seconds (˜20 KHz intrinsic response). There are 25 rows of thenanorods in the pixel to provide adequate row-to-row separation for rowindependence. Thus there are ˜830 gold nanorods contributing toseparation enhanced polarizability, thus signal strength. This array candetect a chosen single polarization. The polarizability of a single goldnanorod in silicon is the same as for structure 1 above. But the nanorodpolarizability is enhanced for the closepacking in the FIG. 15 barrangement of the parallel sets of nanorods with the polarizabilitybehavior as shown in FIG. 15 d. In FIG. 21, it can be seen that theoptimal spacing of the nanorods is ˜3.10⁻⁷ m or 0.3 microns.

The ˜825 coupled nanorod pixel responsivity curve for this singlepolarization ‘enhanced-polarizability’ detector is illustrated in FIG.22, which shows better responsivity performance than the 100 A/Wmicrobolometer, even though only a single-polarization component isbeing detected. Thus, this structure provides a better responsivity forthe single polarization while at the same time providing extrapolarization selectivity.

The comparisons shown here are for illustrative purposes only and do notshow the further superiority of smaller diameter gold nanorods for thisLWIR sensor embodiment.

Thermal Noise, Signal Levels, S/N Ratios, NETD & MRTD Performance

For this section the performance parameters for a 830 nanorod LWIRdetector with 10-micron pixels were calculated.

Thermal Noise

The dominating noise for the electrically detected nanorod detector islikely to be the thermal/Johnson noise current [Saleh, B. E. A., &Teich, M. C., 1991; ‘Fundamentals of Photonics’, p 682, Wiley]. Thenoise-variance is provided by σ²=4 kB T B/R, where k_(B) is Boltzmann'sconstant, T is the temperature, 300° K, B is the circuit bandwidth(about 10⁶ to take advantage of the RC time constant) and R the siliconpixel resistance ˜10⁹ Ohms. For signal to noise calculations, root meansquare (rms) is used, which for a room temperature Si/Au-pixel of 10microns square, yields a equal to be around 1.82 picoAmps.

In practice, it may be desired to trade noise current for a lower pixelresistance. Additional capacitive load from the pixel's circuitry willalso detract from this intrinsic, ideal calculation of σ. In practice, σmay increase ˜×10, so we assume a degradation of ×10 in furthercalculations. σ=18.2 pico-Amps.

Signal Levels

For an image at 300K temperature filling a non BLIP (Background LimitedInfrared Photodector) LWIR optical system operating at f/1 (50%transmitting), the image irradiance is ˜12.5 Wm-2 [RCA, 1968;‘Electro-Optics Handbook’, RCA, Harrison, N.J. 07029, USA. Technicalseries volume EOH-10].

Signal to Noise Ratio

Using the signal and noise values just estimated, the signal to noiseratio, assuming a 10 micron square pixel of responsivity 100 A/W,calculates to be ˜6.89*10⁴. This yields excellent NETD and MRTDperformance. If for engineering reasons σ is 10 times worse thancalculated above, the signal totnoise ratio would fall to ˜6.89*10³,which is still respectable.

A wide dynamic range (gray scale) in temperature space might be expectedfrom such a 8-12 microns LWIR system, which should provide additionaladvantages over microbolometers. A dynamic range exceeding 12 bits,(i.e., >4096) can be calculated based on the ratio of the detector'ssaturation current to dark current.

The main reason for this promising performance is that the dominantnoise component associated with the plasmon based electronic pixel,which arises from thermally induced fluctuations in the motion of theelectrons transferred from the sensing element to the readoutelectronics, is very much smaller than the dominant noise componentassociated with a microbolometer pixel, which arises from thermallyinduced fluctuations in the motion of the phonons transferred from thesensing element to the readout electronics. Further, by reducing thediameter of the gold nanorods from 20 nm to 10 nm, the sensitivityperformance can be reduced by nearly an order Of magnitude.

NETD—Noise Equivalent Temperature Difference

Using the standard NETD equation [Kruse, P. W., 2001; ‘Uncooled ThermalImaging Arrays, Systems, and Applications’, SPIE Tutorial Texts inOptical Engineering, Vol. TT51] for the worst case S/N ratio of 6.89*10³estimated above, NETD in the 8-14 microns band calculates to be 6.9milliKelvins, a substantial improvement over published figures forcirca-1999 microbolometers. Reduction of the diameter of the goldnanorods allows for the design NETD in the sub 1 milliKelvin region,which is better than more current microbolometer performance.

MRTD—Minimum Resolvable Temperature Difference

MRTD is a function of spatial-frequency in the imaging system. Todetermine the MRTD, a standard equation may be used as in known [Lloyd,J. M., 1975; ‘Thermal Imaging Systems’, Plenum Press].

For the purposes of calculation, a f/1 system is assumed with a 10micron pixel whose pixel field of view is ˜1 milliradian. It is furtherassumed an observer's eye is using photopic (cone) vision, looking at abright display, not low light level conditions, so its integration timeis ˜15 milliseconds. It is further assumed a display screen frame-rateof 120 Hz and a detector bandwidth of 100 KHz. A composite MTF of anentire model system was created using a Gaussian squared function whoseshape is as shown in FIG. 23.

On this basis, the MRTD curve appears as shown in FIG. 24; now for adetector whose gold nanorods are only 10 nm diameter. For thiscalculation >80% of spatial-frequency performance would be sub 0.1milliKelvin MRTD; and MRTD for ˜100% of the entire range of spatialfrequencies is less than 1 milliKelvins. Comparison of this MRTD withother LWIR detectors is shown in FIG. 25 which shows the potentialsuperiority of the gold nanorod detectors as described herein.

The detector spectral response and the figure of merit, D* [Lloyd, J.M., 1975; ‘Thermal Imaging Systems’, Plenum Press] may also becalculated. In FIG. 26 an unusual and unexpected spectral response shapeis observed, because of the nanorod resonance detection process insteadof a p-n junction process with its long wavelength cut off at thebandgap.

Detector Device Structures

In addition to the detector device structure shown in FIG. 6,alternative detector structures for the direct electrical determinationof plasmon resonance EMR detection are possible. For example,alternative device architectures and structures are shown schematicallyin FIG. 27 to FIG. 30.

FIG. 27 illustrates a detector where, to enhance the electric fieldacross the nanoparticles, the nanoparticles are embedded in aheterojunction. The heterojunction may be a p-n junction, a n-Njunction, or a metal-semiconductor junction, for example. The embodimentof FIG. 27 has the same structure as that of the embodiment of FIG. 6,except that the nanoparticles 310 are embedded in the heterojunctionbetween the intermediate layer 318 and the material 314, and the samereference numerals are used to denote the same elements.

FIG. 28 illustrates a detector according to an embodiment of theinvention with an orthogonal implementation for current generation,using nanoparticles operated serially, the current generation flows in alateral direction. The detector in FIG. 28 includes a detector material2516 on a substrate 2528, a first electrode 2524, and a second electrode2526, where the first electrode and second electrode function as thevoltage biasing element. The detector material 2516 comprises asubstantially regular array 2512 of nanoparticles 2510 embedded in amatrix material 2514.

In this embodiment the nanoparticles 2510 may be gold nanorods, whichare embedded in the matrix material 2514 which may be n-doped silicon,for example. The substrate 2528 in this embodiment may be a dielectricsubstrate, for example. The detector may also include a dielectricanti-reflection coating 2530 over the detector material 2516 to enhanceabsorption in the detector material 2516.

FIG. 29 illustrates a detector with an inverse nanoparticle operation,where the nanoparticles are the semiconducting material instead of themetal, where the nanoparticles are surrounded by metal, and are disposedin nanoholes in the metal. Again, the physics in play here is that ofmaximizing the direct absorption of the incident EMR into thenanoparticle itself, not the maximized scattering of, or extra-ordinarytransmission of, sub-wavelength apertures in metal surfaces, as used inprevious research [Ebbesen et al, 1998; Ghaemi et al, 1998].

To calculate the pertinent performance parameters for the FIG. 29design, the real and imaginary refractive index and complex permittivity(dielectric function) values of the metal and semiconductor may beexchanged thereby obtaining the plasmon-absorption directly into themetal surrounded semiconductor nanoparticles. This is the inversegeometry of what has been described with respect to the detectors ofFIGS. 6, 27 and 29.

The detector in FIG. 29 includes a detector material 2616 on a substrate328, a bottom electrode 326, where the bottom electrode 326 and topelectrode 2630 functions as the voltage biasing element. The detectormaterial 2616 comprises a substantially regular array 2612 ofnanoparticles 2610 embedded in a matrix material 2614. In thisembodiment the nanoparticles 2610 may be silicon nanorods, which areembedded in the matrix material 2614 which may gold.

As shown in FIG. 29, laterally surrounding the detector material 2616 isa surface passivation layer 322, which may be an oxide layer about >0.1micron thick. The substrate 328 may have an intermediate layer 318,which may comprise n-doped silicon, and which may have a thickness, forexample, of about two microns. The substrate 328 may also have a lowerlayer 320, which may be a n⁻ or n⁺ doped <100> or <111> silicon layerfor example.

FIG. 30 illustrates a detector where nanoparticle absorber arrays areused to replace the standard absorption region of an avalanchephotodiode to improve its performance. The standard π-region is replacedby metal nanoparticles for carrier creation prior to employing the p-navalanche effect on the photoelectrons. This design is especiallyadvantageous for single-photon counting enhancement using very weak,‘photon-starved’ light fields.

The detector in FIG. 30 includes a detector material 2716 in a substrate2728, a top electrode 2724, and a bottom electrode 2726, where the topelectrode and bottom electrode function as the voltage biasing element.The detector material 2710 comprises a substantially regular array 2712of nanoparticles 2710 embedded in a matrix material, i.e., the π-regionof the substrate 2728, which may be doped silicon. In this embodimentthe nano-particles 2710 may be gold nano-rods. The detector in FIG. 30further includes in order from the top electrode 2724 to the substrate2728 a n-region 2732 and a p-region 2734, which may be doped silicon asis known for avalanche photodiode structures. As shown in FIG. 30,laterally surrounding the n-region 2732 and p-region 2734 is a surfacepassivation layer 2722.

Short Wavelength Infra Red (SWIR) Detectors

The nanoparticle detector may also be arranged to preferentially detectEMR with wavelengths in the SWIR region. In this embodiment there isdescribed the performance of a nanoparticle detector designed to operatein the SWIR region, usually accepted to be between 1 micron and 3microns in wavelength. For example, gold nanorods may be used with aconfiguration such as that in the detectors and pixel arrangements ofFIGS. 6, 15 a-15 b and 27-30.

The nanorod length may be varied for peak detection wavelength, in asimilar fashion to LWIR detection described above, as is shown in FIG.31. FIG. 32 illustrates that a 40 nm long, 10 nm diameter nanorodarrangement has peak absorption at a 2 microns SWIR wavelength.

FIG. 33 illustrates the SWIR-APD (FIG. 30 structure) gained responsivityfor 6600 nanorods in a 10 micron square pixel, with a system with goldnanorods in a silicon matrix. As can be seen in FIG. 33, the curvescompare well to the responsivities from InGaAs photodiodes at ˜2 micronsand 2.65 microns, respectively.

FIG. 34 illustrates the enhanced absorption of the array of 6600 goldnanorods used for determining the FIG. 33 curve for a nanorod latticespacing of ˜150 nm and is nearly 4 orders of magnitude greater perparticle than the absorption of a single nanoparticle not placed in alattice array as shown in FIG. 34.

The detector performance of such an array of gold nanoparticles for SWIRdetection was calculated as follows for f/1 optics (T=50%) using SWIR of2.6 microns wavelength, with 10 micron square pixels. The APDresponsivity for the FIG. 25 structure detector structure equals 1000A/W. This yields the following performance predictions from simulations.Under conditions of Magnitude +2 starlight conditions (one tenth ofquarter-moonlight) the signal to noise ratio in each pixel is S/N ˜13.6and its dynamic range ˜5685, based on saturation/noise current ratio. Ifthe detector bandwidth is set to 10⁵ Hz, then the noise-equivalent poweris NEP ˜8.6×10⁻¹⁴ Watts which yields a specific detectivity D*˜3×10¹²Jones, which nearly falls on the ideal photovoltaic D* curve for atemperature of 295K.

FIG. 35 illustrates the D* curve compared to an ideal photovoltaicoperating at 2π steradians—and 295K temperature. Unusual and unexpectedshapes are observed, because of the nanorod resonance detection process,instead of a p-n junction process with its long wavelength cutoff at thebandgap that is typically observed for detector performance. Theextended long wavelength performance can be very useful in night-visionimaging.

Visible (VIS) and Near Infra-Red (NIR) Region Detectors

The nanoparticle detector may also be arranged to preferentially detectEMR with wavelengths in the Visible/Near-Infra-Red (VIS-NIR) region. Inthis embodiment there is described the performance of a nanoparticledetector designed to operate in the VIS-NIR region. The detectors andpixel arrangements of FIGS. 6, 15 a-15 c and 27-30 may be used for suchdetectors. The nanoparticles may be gold, for example, or another noblemetal, transparent oxides such as Al:ZnO, Ga:ZnO or ITO, transitionmetaloxides such as TiN or ZrN, or possibly graphene.

An example of VIS and NIR detection is provide with 22 nm diameter goldnanorods embedded in silicon, where the illumination is assumedperpendicular to the gold nano-rod lattice plane. As seen in FIG. 36, apeak absorption region for EMR with wavelengths 450 nm, 500 nm, 615 nm,700 nm, 800 nm and 1-micron is shown, where the peaks occur at differentnanorod lengths. FIG. 37 illustrates the calculated responsivities of a10-micron detector element comprising ˜5000 22 nm diameter gold nanorodsin a close-coupled array as a function of nanorod length, with theresponsivity of 55 nm length nanorods shown for comparison.

As can be seen in FIG. 37, there is a peak responsivity of around 100A/W for 55 nm long gold nanorods illuminated at 1 micron NIR wavelength.For 615 nm wavelength EMR, the peak in responsivity occurs at a lengthof 7 nm with 10× the responsivity. With this arrangement there is nodiscernable peak response for wavelengths less than about 550 nm. In the450 nm and 500 nm curves there is evidence of Mie absorptionoscillations for very short nanorods, as expected, because Rayleigh-Mielight-scattering is dominating here—with such small particles, notlight-absorption. The optimum center-to-center spacing of the goldnano-rods was calculated and is estimated to be approximately 0.3microns, or 300 nm to give maximum single particle absorption from theenhanced-lattice-response effect.

LWIR Region Detectors with SiC Nanoparticles

In this embodiment, for LWIR region detection, silicon carbide particlesare used as an alternative to gold for the nanorod array materialembedded in a semiconductor such as silicon.

The underlying physics of the nano-absorption in this embodiment is thatof phononics instead of plasmonics. Phononics is another polaritonmechanism, and the math behind the calculations for absorbtivity is verysimilar to the localized plasmon case discussed above with respect togold nanoparticles. With phononics, the effect of absorption of EMR isto heat the detector, where the absorbed EMR photon is convertedultimately to a phonon and thus heat, so the same limitations of thermaltime constant apply here as they do for a microbolometer.

The LWIR absorption for this embodiment may be calculated in acorresponding fashion to the LWIR absorption based on plasmonics. As astart, FIG. 38 a illustrates the complex refractive index curves forsilicon carbide material in the LWIR region, while FIG. 38 b illustratesthe corresponding polarizability curve for a 10 nm silicon carbidesphere also in the LWIR region. FIG. 39 illustrates the LWIR absorptionof 20, 50, 100, 200 and 400 nm diameter SiC spheres.

As can be seen, the width of the detector's resonance peak curve isnoticeably reduced within the LWIR. This feature allows for the designand manufacture of detectors selective to very specific wavelengthswithin the LWIR, while having the same polarization advantages asdescribed in FIGS. 15 a-15 c for arrays of such nanoparticles. Theresonances are confined to a spectral region far smaller than can beobtained with the typical multi-layer dielectric interference filterapproach. The resonance peak may be changed by changing the matrixmaterial surrounding nanoparticles, such as the semiconductor materialand its doping-levels and type.

Terra-Hertz and Microwave Region EMR Detection

The nanoparticle detector may also be arranged to preferentially detectEMR with wavelengths in the Terra-Hertz and microwave region. InSb,gold, silver or lead nanoparticles, for example, may be formed into anarray, which is embedded in silicon or some other suitable semiconductorfor direct electrical detection of incident THz EMR. Detector geometriesand pixel arrangements of FIGS. 6, 15 a-15 c and 27-30 apply.Applications for a Terra-Hertz detector in the wavelength range of 01.to 10 THz, may include, for example, a camera, such as for example a SLRcamera. Such THz detectors provide advantages such as imaging throughsolid materials.

The essential underlying physics for THz detection is that of thecomplex dielectric constant for metals and semi-conductors. Frompermittivity equations for permittivity ∈, the real, n, and imaginary,k, refractive index values may be calculated as follows:

$\overset{\sim}{\varepsilon} = {{\varepsilon_{1} + {i\;\varepsilon_{2}}} = \left( {n + {ik}} \right)^{2}}$

-   -   Conversion between refractive index and dielectric constant is        done by:

$\begin{matrix}{\varepsilon_{1} = {n^{2} - k^{2}}} \\{\varepsilon_{2} = {2{nk}}} \\{n = \sqrt{\frac{\sqrt{\varepsilon_{1}^{2} + \varepsilon_{2}^{2}} + \varepsilon_{1}}{2}}} \\{k = \sqrt{\frac{\sqrt{\varepsilon_{1}^{2} + \varepsilon_{2}^{2}} - \varepsilon_{1}}{2}}}\end{matrix}$

The response of an array of nano-structures may be used to estimate theresponsivity of such nanostructures made of metals and semiconductorsembedded in air or semiconducting materials.

A good THz detector may be achieved, for example, using a regular arrayof 120 10 micron diameter gold cylinders embedded in a silicon matrix,where the pixel size is 1 mm square, and the array is linear. For a rodlength of 250 microns, the responsivity can be calculated to be 2.5 A/Wat 0.3 THz for a 500 mW peak power THz laser source in a 4 inch f/2optical system.

The responsivity may be increase even further for a array of 5 rodsspaced 200 microns across with a rod length of 1 mm to 35 A/W for a 100mW peak power THz laser source in a 4 inch f/2 optical system. Theresponsivity of this detector configuration with the load-resistor of50K Ohms corresponds to a responsivity in Volts/Watt of 1.7×10⁶, whichexceeds the responsivity figure of comparable pyro-electric detectorsfor THz detection by more than 10×. For the noise-level of thisdetector, we calculate a worst-case NEP of 8×10⁻¹³ Watts per root-Hertz,and D* of 2.4×10¹² Jones.

Vitamin Enhancement of IR Region Photodetection

The nanoparticle detector may also be arranged to preferentially detectEMR with wavelengths in the IR region based on vitamin enhancement. Sucha detecting system can be based on a chemical component that has acrucial role in photosynthesis, which provides a most efficient processfor converting light into a flow of electrons. The process of convertinglight into electrons is described, in [Miyachi et al, 2010; ‘Aphotosensing system composed of photosystem I, molecular wire, goldnano-particle, and double surfactants in water’, Chem. Commun. 46, pp2557-2559], which describes enhanced photodetection using goldnanoparticles and surfactants linking them to a gold film. FIG. 40,taken from Miyachi, illustrates the operation schematically. WhileMiyachi disclose photodetection of visible wavelengths using 2 nm goldparticles, such photodetection could possibly be extended into the SWIR& LWIR regions.

In plants, photosynthesis begins when a photon of light frees anelectron from a chlorophyll molecule. A device based on this process ispotentially sensitive enough to detect even a single photon, making ituseful as a light sensor. The devices uses molecules similar to vitaminK1. Nanoparticles are an important part of the system, acting as a storefor electrons. This system could be extended out to the SWIR and LWIRregions to capture the high efficiency potential by use of differentvitamins to replace vitamin K1.

Multi-Spectral Band and Polarization Geometries

This embodiment provides nano particle arrays which allow for thedetection of multiple wavelengths simultaneous with the same detector.Such multiple wavelength detection can provide advantages in focal planearray (FPA) imaging systems, with the special advantage that two or morewavelengths may be detected simultaneously in the same detectorstructure. In this regard, the nanorods in different portions of a pixelmay be made to have different lengths and aspect ratio that provide aresonance at different specific wavelengths, and/or nanorods may bearranged to detect different polarizations of interest.

An exemplary nanorod array geometry for this purpose is illustrated inFIG. 41. The detectors of FIGS. 6 and 27-30 apply here as basic detectorstructures. FIG. 41 shows a possible nanorod layout. FIG. 41 showsarrays of differently oriented, and different length, nanorods for thepurposes of wavelength and orthogonal polarization selectivity within asingle pixel detector. This structure could be applied across afocal-plane array detector to assist in detection of specific featuresin the scene being observed.

For example, a system may be deployed for simultaneous SWIR (˜0.3microns full width half maximum (FWHM)) and LWIR (˜5.5 microns FWHM)detection. FIG. 42 illustrates these relative spectral bandwidths thatmay be obtained in those two spectral regions.

Solar Cells

With respect to solar cells, direct electron detection as describedherein may be used in place of (or together with) the plasmon scatteringeffects currently used in nanoparticle array light scatterers to enhanceabsorption in optical modes in silicon, where detector geometries andpixel arrangements of FIGS. 6, 15 a-15 c and 27-30 apply as solar-celldetector structures.

The invention claimed is:
 1. A detector material for detectingelectro-magnetic radiation, comprising: a substantially regular array ofnano-particles embedded in a matrix material; and the nano-particlesarranged such that when a bias voltage to the matrix material isapplied, electrical current is directly generated based on a cooperativeplasmon effect in the detector material when electro-magnetic radiationin a predetermined wavelength range is incident upon the detectormaterial, where the dominant mechanism for decay in the cooperativeplasmon effect is non-radiative.
 2. The detector material of claim 1,wherein the nano-particles comprise a metal, and the matrix materialcomprises a semiconductor material.
 3. The detector material of claim 2,wherein the nano-particles comprise gold.
 4. The detector material ofclaim 2, wherein the matrix material comprises one of silicon or InSb.5. The detector material of claim 2, wherein the predeterminedwavelength range comprises at least one of the visible, near infra-red,short-wave infrared, medium-wave infrared, infra-red, long-waveinfra-red, and tera-hertz regions of the electromagnetic spectrum. 6.The detector material of claim 1, wherein the nano-particles comprise asemiconductor material, and the matrix material comprises a metal. 7.The detector material of claim 1, wherein the nano-particles comprise afirst semiconductor material, and the matrix material comprises a secondsemiconductor material.
 8. The detector material of claim 7, wherein thesecond semiconductor material is highly doped.
 9. The detector materialof claim 1, wherein the nano-particles comprise a semiconductormaterial, and the matrix material comprises a conductive contactmaterial having ohmic or Schottky barrier properties with respect to theembedded nano-particles.
 10. The detector material of claim 1, whereinthe matrix material comprises a hetereojunction, and the nano-particlesare embedded in the heterojunction.
 11. The detector material of claim1, wherein the matrix material comprises a metal-semiconductor junction,and the nano-particles are embedded in the metal-semiconductor junction.12. The detector material of claim 1, wherein the matrix materialcomprises a metal as a perforated contact electrode and thenano-particles comprise a semiconductor material.
 13. The detectormaterial of claim 1, wherein nano-particles are cylindrical, spherical,cubic, rectangular-cubic, ellipsoidal, planar or spiral-twisted.
 14. Thedetector material of claim 1, wherein the nano-particles have a longaxis, and the array comprises one or more pixels.
 15. The detectormaterial of claim 14, wherein the nano-particles in each pixel arearranged such that the long-axis of each nano-particle in the pixel areoriented along a same direction.
 16. The detector material of claim 14,wherein the nano-particles in each pixel comprise a first group ofnano-particles and a second group of nano-particles, the long-axis ofeach nano-particle in the first group oriented along a first direction,the long-axis of each nano-particle in the second group oriented along asecond direction.
 17. The detector material of claim 16, wherein thefirst direction is perpendicular to the second direction.
 18. Thedetector material of claim 16, wherein the nano-particles of the firstand second groups are arranged in the same region of the pixel to form achecker-board pattern.
 19. The detector material of claim 16, whereinthe nano-particles of the first and second groups are arranged indifferent regions of the pixel.
 20. The detector material of claim 16,wherein the nano-particles of the first group are sensitive to apolarization of the electromagnetic radiation in the predeterminedradiation range in a first polarization direction, and thenano-particles of the second group are sensitive to a polarization ofthe electromagnetic radiation in the predetermined radiation range in asecond polarization direction.
 21. The detector material of claim 14,wherein the nano-particles in each pixel comprise a first group ofnano-particles arranged in a first region of the pixel and a secondgroup of nano-particles arranged in a second region of the pixel,wherein the nano-particles in the first group are arranged in the firstregion to optimally detect electromagnetic radiation in a firstwavelength range, and the nano-particles in the second group arearranged in the second region to optimally detect electromagneticradiation in a second wavelength range different from the firstwavelength range.
 22. The detector material of claim 21, wherein thenano-particles in each pixel further comprise a third group ofnano-particles arranged in a third region of the pixel, thenano-particles in the third group are arranged in the third region tooptimally detect electromagnetic radiation in a third wavelength rangedifferent from the first wavelength range and the second wavelengthrange.
 23. The detector material of claim 14, wherein the array ispolarization sensitive to the electromagnetic radiation in thepredetermined radiation range.
 24. The detector material of claim 1,wherein the predetermined wavelength range comprises multiplepredetermined wavelength subranges.
 25. The detector material of claim24, wherein the predetermined wavelength subranges are separated fromeach other.
 26. The detector material of claim 25, wherein thepredetermined wavelength subranges are continuous with each other. 27.The detector material of claim 1, wherein the predetermined wavelengthrange is between the deep ultra-violet and microwave regions of theelectromagnetic spectrum.